The present invention relates, in general, to semiconductor devices and, more particularly, to semiconductor-on-insulator devices operating in a fully-depleted mode.
Semiconductor-on-insulator (SOI) devices are typically formed using silicon and generally operate in either a partially-depleted mode or a fully-depleted mode. SOI devices operating in a fully-depleted mode exhibit very low sub-threshold slope and do not suffer from the so-called kink effect, however, such devices have a large threshold voltage sensitivity to the thickness of the thin-silicon film used for manufacture (for example, this sensitivity is typically 100 mV/100 angstroms), and require thin silicon films having a thickness of about 800 angstroms. These two disadvantages are significant because such a thin silicon film is difficult to manufacture with uniformity. Thus, the large threshold voltage sensitivity leads to non-uniform threshold voltages for the manufactured devices.
In contrast to the fully-depleted mode, SOI devices operating in a partially-depleted mode have reduced threshold voltage sensitivity to the thickness of the thin silicon film and are easier to manufacture because the thickness of the thin film can be greater than that required for fully-depleted devices. However, partially-depleted devices exhibit the kink effect, which adversely affects their use in analog applications. They also exhibit a degradation of sub-threshold slope compared to devices operating in the fully-depleted mode. A degraded sub-threshold slope increases device leakage when the device is turned "off".
When SOI devices are used for low power integrated circuits, such as in cellular phones and pagers, it is desirable that the sub-threshold slope be very low to reduce standby power consumption. Accordingly, there is a need for an SOI device that operates in a fully-depleted mode, yet has reduced threshold voltage sensitivity to the silicon film thickness and that can be formed using silicon films having a thickness greater than that required for prior fully-depleted devices.